To get a prompt which indicates Git-branch in Zsh

__git_ps1 is from git-completion.bash. In zsh you probably have to provide your own function to determine the current directories git branch. There are quite a few blog posts about a git prompt for zsh.

You just need:

• a function to provide the branch name
• enable prompt (command) substitution
• add the function to your prompt

For example

git_prompt() {
 ref=$(git symbolic-ref HEAD | cut -d"https://stackoverflow.com/" -f3)
 echo $ref
}
setopt prompt_subst
PS1=$(git_prompt)%#
autoload -U promptinit
promptinit

Update: use the zsh vcs_info module instead of git_prompt()

setopt prompt_subst
autoload -Uz vcs_info
zstyle ':vcs_info:*' actionformats \
    '%F{5}(%f%s%F{5})%F{3}-%F{5}[%F{2}%b%F{3}|%F{1}%a%F{5}]%f '
zstyle ':vcs_info:*' formats       \
    '%F{5}(%f%s%F{5})%F{3}-%F{5}[%F{2}%b%F{5}]%f '
zstyle ':vcs_info:(sv[nk]|bzr):*' branchformat '%b%F{1}:%F{3}%r'

zstyle ':vcs_info:*' enable git cvs svn

# or use pre_cmd, see man zshcontrib
vcs_info_wrapper() {
  vcs_info
  if [ -n "$vcs_info_msg_0_" ]; then
    echo "%{$fg[grey]%}${vcs_info_msg_0_}%{$reset_color%}$del"
  fi
}
RPROMPT=$'$(vcs_info_wrapper)'